Non-radiating autodyne frequency converter



April 19, 1960 D. A. s. DRYBROUGH 2,933,599

NON-RADIATING AUTODYNE FREQUENCY CONVERTER Original Filed March 2'7.1956 l3 2, i l? INTERMEDIATE- O FREQUENCY AMPLIFIER O 0 P4 DETECTOR ANDA.v.c.

SUPPLY O O l' n IO RADIO- 0 AMPLlFlERo AUDIO o- FREQUENCY g AMPLIFlER 0FIG.]

Patented Apr. 19, 1960 NON-RADIATING AUTODYNE FREQUENCY CONVERTER DavidA. S. Drybrough, Coventry, England, assignor to Hazeltine Research,Inc., Chicago, 11]., a corporation of Illinois Continuation of abandonedapplication Serial No. 574,151, March 27, 1956. This application June25, 1958, Serial No. 744,384

Claims priority, application Great Britain April 5, 1955 a 1 Claim. (Cl.25020) General This invention relates to frequency changers orconverters.

This application is a continuation of applicants prior application,Serial No. 574,151, filed March 27, 1956.

The invention is concerned in particular with frequency changersof thekind comprising an electron-discharge device such as a pentode ortetrode valve connected in circuit to operate as a self-oscillatorymixer, the oscillations generated in the valve or tube being sustainedby means of a tunable parallel-resonant circuit connected between thescreen and control electrodes 'of the valve and having an intermediatepoint connected to the cathode of the valve by a path'which is of lowimpedance at the oscillation frequency.

Where a frequency changer is used for high-frequency operation, forexample where the oscillation frequency of the order of 100 mc.,difiiculties may arise due to radiation at the oscillation frequency. Inparticular, where such a frequency changer is used as the first detectorin a superheterodyneradio receiver, an appreciable voltage at theoscillation frequency may be fed back 'to the aerial through theradio-frequency circuits. Back radiation of energy at the oscillationfrequency is undesirable since it creates interference which impairs theoperation of nearby equipment. In view of the interest of the FederalCommunications Commission in reducing such interference, radio andtelevision manufacturers are continually seeking means for reducing theextent of such interference.

It is an object of the present invention to provide a new and improvedfrequency changer in which such difficulties are alleviated.

g It is another object of the invention to provide a new and improvedfrequency changer which is particularly adapted for use at highfrequencies such as those encountered in frequency-modulation broadcastreceivers.

It is a further object of the invention to provide for use at highfrequencies a new and improved frequency changer which avoids the needfor neutralizing circuits.

It is an additional object of the present invention to provide a new andimproved frequency changer which is-relatively simple in constructionand inexpensive to manufacture.

In accordance with the invention, a high-frequency wave-signal frequencychanger having minimum back radiation comprises an electrical Valveincluding a corrent-emitting electrode, at least one current-receiving,electrode, and a control electrode interposed therebetween and an,input circuit having a low impedance at the heterodyne-signal frequencyand a beat frequency and including means coupledbetween the emittingelectrode of the valve and a point of fixed high-frequency potential forapplying therebetween a Wave signal. The frequency changer alsocomprises a heterodyne tuned circuit e'eu na to a current-receivingelectrode and'to th'e'control electrode of the valve through a pathhaving a low impedance at the heterodyne frequency and having anintermediate terminal connected to the point of fixed Fig. 1 is acircuit diagram, partly schematic, of a 7 complete wave-signal receiverwhich includes a frequency changer in accordance with the presentinvention in a particular form; and

- Fig. 2 is. a circuit diagram of a frequency converter in accordancewith a modified form of the present invention.

General description of Fig. 1 receiver Referring now more particularlyto Fig. 1, there is represented a complete superheterodyne receiverwhich includes a radio-frequency amplifier 10 having its input circuitconnected to an antenna 11 and its output circuit connected by way ofthe adjustably tuned primary winding 29 of a transformer 17 to afrequency changer or oscillator-modulator 12 in accordance with one formof the present invention. Connected in cascade with the frequencychanger 1.2, in "the order named, are an intermediatqfrequency amplifier13 of one or more stages, a detector and automatic-voiume-control supply14, an audio-frequency amplifier 15 of one or more stages, and a, soundreproducer 16.- The detector of unit 14 maybe of any convenient formsuch as one for deriving either.

the modulation components of an amplitude-modulated wave signal or of afrequency-modulated Wave signal.

For the embodiment of the invention under consideration, however, unit14 will be considered to-include a detector for frequency-modulated Wavesignals. An automatic-amp]ification-control bias derived from. the AVGjust described, with the exception of the frequency converter 12, may beof conventional construction and operation, the details of which arewell known in the art, rendering detailed. description thereofunnecessary.

General operation of receiver of Fig. 1 Considering briefly theoperation of the receiver as a whole but neglecting for the moment thedetails of the detector 14, thereby deriving the audio-frequencymodulation components and the ABC bias potential. The modulationcomponents last mentioned are, in turn, amplified in the audio-frequencyamplifier 15 and are reproduced by the sound reproducer 16. The. biaspotential is employed as previously stated.

Description of frequency'changer-IZ of Fig. 1 One arrangement inaccordance with the invention .mediate frequency of 10.7 me.

includes an electron-discharge device such as as a pentode valve 20arranged to operate as a' self-os'cillatory mixer, that is, as both amixer and a local oscillator, the local oscillations being generated bymeans of g a parallel resonant feedback circuit 21 connected between thescreen electrode 22 and control electrode 23 of the valve, the controlelectrode being biased negatively by means of the conventional capacitorgrid-leak combination 24,

The frequency changer I tween the screen and control grids of thepentode is substantially balanced with respect to earth; the bifilarwinding of the inductor 27 in the feedback circuit ensures that thebalanced condition of the circuit is maintained over the wholeoscillator tuning range, since movement of the tuning core 28 does notafiect the balance of the circuit. Furthermore, the impedance of thesignal input circuit is always low (of the order of 100 ohms) throughoutthe normal frequency range of the local oscillation. By virtue of thesetwo facts, the voltage generated across the signal'input circuit at thelocal oscillation frequency is always of small amplitude, and thusrelatively little 25. The parallel-resonant circuit 21 includes a fixedcapacitor 26 and an inductor 27, the'inductance of which i is variableby movement of a ferromagnetic core" 28 so as to tune the circuit, thetuning'core of the oscillator circuit being ganged with a similar corein the tuned primary winding 29 of the radio-frequency circuit so thatthe local oscillation frequency is always 10.7 mc. above the frequencyof the received signal. For best results a bifilar winding is utilizerfor the inductor. Thecenter point or first terminal 40' of the inductor27 is connected via a high value resistor 30 to the positive terminal ofa constant voltage source indicated as +B whose negative terminal isearthed, that is, connected to a point of fixed reference potential.This first terminal is also connected to earth via a capacitor 31 whoseimpedance is low at at least one of the oscillation or heterodynefrequency and the intermediate or beat frequency. The second terminal 41of theinductor 27 is connected to the contrdol electrode 23 throughcondenser 24 while the third terminal 42 is directly connectedtothescreen electrode 22. i 1

The signal input circuit for'the frequency changer presents a lowimpedance'to heterodyne and beat-frequency wave signals and includesmeans coupled between the current-emittingelectrode or cathode of tubeand ground for applying the radio-frequency signal therebetween. Thislast-mentioned means comprises an inductor 32 and a capacitor 33connected in parallel between the cathode 40 of the pentode and earth,the inductor being tightly coupled to the inductor 29 of the transformer17 in the tuned radio-frequency circuit to which the received signal isapplied. The values of the inductor 32 and the capacitor 33 in thesignal input 'circuitof the frequency changer are chosen so that thiscircuit resonates at a frequency of approximately 86 mc., the circuithaving a low Q by virtue of the damping produced by the cathode inputadmittance of the pentode '20 and thus providing an adequate signalthroughout the signal frequency band. The suppressor electrode 34 of thepentode is connected to earth, and the anode 35 of the pentode isconnected via a circuit 36 tuned to the intermediate frequency to thepositive terminal of the constant voltage source +B'.

Explanation of operation of frequency changer 12 of Fig. 1

In operation of the frequency changer 12, an input signal fed to thecathode of the pentode is, by virtue of the nonlinear operation of thevalve, heterodyned with the oscillation produced in the pentode so thatan intermediate-frequency signal appears at the control electrode 23 ofthe pentode; this signal is amplified by the pentode, good amplificationbeing obtained by virtue of the fact that the screen electrode isvirtually earthed at the intermediate frequency, and anintermediate-frequency output signal is derived from the anode 35 of thepentode. r f

In operation, the feedback circuit 21 connected bepower 'is'fed back atthe oscillation frequency to the radio-frequency circuit. Directcoupling between the other parts of the frequency changer circuit andthe radio-frequency circuit is eliminatedby means ofthe suppressor orshield electrode 34 serving as an' earthed electrostatic screen disposedbetweenthe frequency changer and the radio-frequency circuit, theinductor 32 in the signal input circuit of the frequency changer beingdisposed on the same side of this screen as the radiofrequency circuit.

The use of the bifilar winding for the inductor 27 in the feedbackcircuit 21 also results in a tight coupling between the two halves ofthe inductor, so that the unidirectional potential applied to the screenelectrode 22 may be made comparatively low while still maintainingoscillation, thus ensuring that the total power developed at the localoscillation frequency is relatively small and further reducing the riskof radiation at the oscillation frequency. Operation is further enhancedby the fact that the signal input circuit has a. capacitative impedanceat allfrequencies within the normal frequency range of the localoscillation, thus tending to reduce the input admittance of thelocaloscillator and make it easier to maintain oscillation with a relativelylow unidirectional potential applied to the screen electrode.

The use of a pentode in the frequency changer 12 has the advantage thatthe output'impedance of the valve 20 is high, so that thereis littledamping of the intermediate-frequency output circuit 36, and there isalso little back coupling through the valve at the intermediatefrequency, thus removing the necessity for neutralization at thisfrequency. r

From the foregoing description and explanation of the operation of thefrequency changer 12 of Fig. 1, it will be clear that the pentode 20comprises an electrondischarge device having a single space currentpath; that the cathode 40 thereof constitutes the current-emittingelectrode; the screen electrode 22 constitutes a first current-receivingelectrode; the suppressor electrode constitutes a shield electrode; andthe anode constitutes a second current-receiving electrode. It will alsobe clear that the condensers 31 and 33constitute means having a lowimpedance for the heterodyne and beat-frequency signals and serve tocouple terminal 40 to the cathode 40 of the pentode.

Description of Fig. 2 frequency changer 12 Referring now to Fig. 2,there is represented a frequency changer which is substantially the sameas that of Fig. 1. Accordingly, corresponding elements are designated bythe same reference numerals. The frequency changer of Fig. 2 difiersfrom that of Fig. 1 only in the manner of making circuit connections tothe anode and suppressor circuits of the pentode 20. The anode 35 isenergized from a source +B connected to the anode through a resistor 52in series with a winding 51 of the tuned circuit 36'. Thus the anode 35and the screen electrode 23 are energized from the same source but atdifferent potential levels.

The potential supplied to the screen electrode 22 is considerablylowerthan that applied to the anode of the pentode. In a practicalembodiment of the invention, volts were applied to the anodewhile but 50volts were applied to the screen electrode. A by-pass condenser 53 isconnected between the junction of the resistors 30 and 52 and ground.The suppressor electrode 34 of the pentode not only is grounded but isalso connected to the output terminal of the frequency changer 12through a winding 50 inductively coupled to the winding 51 in the anodecircuit of the pentode. The bifilar winding comprising the inductor 27has been represented as comprising a coil or winding portion 54represented by the solid-line construction and another winding portion55 represented by the broken-line construction. It will be noted thatthe portions of the bifilar winding are sen'es connected with twoopposite terminals of winding portions 54 and 55 connected to the firstterminal 40, the remaining terminal of winding portion 55 beingconnected to the second terminal 41, and the remaining terminal ofwinding portion 54 being connected to the third terminal 42.

The operation of the frequency changer 12 of Fig. 2 is essentially thesame as that of Fig; 1 and hence need not be repeated.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed to cover all such changes and modifications as -fall within thetrue spirit and scope of the invention.

What is claimed is:

A high-frequency wave-signal frequency changer having minimum backradiation comprising: an electrical valve including a current-emittingelectrode, at least one current-receiving electrode, and a controlelectrode interposed therebetween; an input circuit having a lowimpedance at the heterodyne-signal frequency and a beat frequency andincluding means coupled between said emitting electrode and a point offixed high-frequency potential for applying therebetween a Wave signal;a heterodyne tuned circuit coupled to a current-receiving electrode andto said control electrode through a path having a low impedance at saidheterodyne frequency, said tuned circuit including a two-partseries-connected bifilar winding with the junction thereof connected tosaid point of fixed potential through a path of low impedance at saidheterodyne frequency, said tuned circuit being electrically balancedwith respect to said point of fixed potential; adjustable meanscooperating with said winding to tune said tuned circuit over a range offrequencies Without disturbing the balance thereof; and a beat-frequencytuned circuit coupled between said current-emitting andcurrent-receiving electrodes.

References Cited in the file of this patent UNITED STATES PATENTS2,107,393 Schlesinger Feb. 8, 1938 2,582,683 Dammers Jan. 15, 19522,662,171 Cock Dec. 8, 1953 2,789,213 Marks Apr. 16, 1957

